Re: [virt-tools-list] Default use of caching on raw volumes

From: Mark van Walraven <markv netvalue net nz>

To: virt-tools-list redhat com

Subject: Re: [virt-tools-list] Default use of caching on raw volumes

Date: Thu, 19 Aug 2010 11:43:11 +1200

On Wed, Aug 18, 2010 at 09:50:23AM -0600, Jamin W. Collins wrote:
> On 08/18/2010 09:23 AM, Daniel P. Berrange wrote:
>>
>> Previously when anyone has suggested setting cache=none for QEMU,
>> other people have appeared with test results showing that other
>
> Care to provide references? I've yet to find a single report where
> caching provides better performance with VMs stored on raw volumes
> except for the use of writeback which is not recommended for anything
> where data integrity is concerned. My suggested change improves
I'll chip in. I observe writethrough on a block device provided by
dm-multipath on top of open-iscsi is faster than cache=none for my usage.
These are, I think, the reasons:
I don't use an iSCSI HBA with a RAM cache, so the qemu cache gets
to do something useful.
A large proportion of my guests have misaligned filesystems within
their virtual disks (usually a 63 sectors/track geometry), which
causes 1KB reads/writes to use two iSCSI operations.
The physical drives have less traffic once the cache is warm,
reducing contention between guests.
The down-side of writethrough is that live migration almost certainly
corrupts data on a virtual drive. So I want to be able to choose
the caching mode per guest. (Ideally, from my point of view, live
migration should be made safe with caching, or disabled when caching
makes it unsafe.)
Regards,
Mark.